There is considerable interest for photodynamic therapy (PDT) as a new treatment modality in cancer patients. PDT consists of systemic administration of photosensitizing drugs, e.g., hematoporphyrin derivative (Hpd) or Photofrin II (enriched in hydrophobic porphyrins in Hpd), allowing them to accumulate in tumors, and exposing lesions to photoradiation (laser irradiation at 630 nm). Encouraging clinical results are seen in breast, lung, bladder, and head and neck tumors; controlled (protocol) studies are underway for bladder and lung cancers. Nevertheless, much research remains to elucidate the mechanisms producing tumor regression. There is a need to define the sites and events leading to cytotoxicity, the porphyrin species causing photosensitization, and identify and refine factors that can impact on optimization of therapeutic response. We are studying two well-differentiated rodent mammary tumor models, which we characterized extensively in vivo and in vitro. In previous studies, we developed an in vivo-in vitro protocol, using biochemical parameters as indicators of photosensitization, to assess sites of action; in this model, endogenous metabolism and its effect on pharmacokinetics occur, providing an opportunity to extend findings from studies in vitro. We now have considerable support for our thesis tht mitochondria are a critical site of action, with impaired ATP production leading to cellular demise. We propose to expand and extend these interesting findings to gain insight on factors enhancing target organ photosensitivity. Experiments are proposed to: 1) ascertain the sites of action and photosensitizing properties of Photofrin II in the functional in vivo-in vitro assay and study ATP levels in situ by NMR spectroscopy; 2) further define mitochondrial sites of action relative to mitochondrial topography; 3) evaluate in depth the role of oxygen as a factor for photosensitization, its effect on '02 production and resulting damage; 4) alter membrane lipid composition by dietary means as one way to enhance photosensitization; 5) assess photosensitivity as influenced by hormonal perturbations that affect tumor growth, e.g., estrogens, ovariectomy, etc., and metabolism such as thyroxine; and 6) initiate studies of selected perturbations for additivity of enhancing photosensitization. Results should provide new insight regarding mechanisms of actions leading to cytotoxicity and could lead to development of combined maneuvers to improve the efficacy of PDT.